1. Field of the Invention
The present invention relates to a method of measuring a surface profile using an atomic force microscope for observing a sample surface by making use of interatomic forces existing between a probing tip and the sample surface.
2. Description of the Prior Art
Recently, development has proceeded on an atomic force microscope (hereinafter referred to simply as an AFM) which is used as an instrument capable of observing the surface of a solid body on an atomic scale.
The principle of the AFM is explained hereinafter with reference to FIG. 5.
In order to detect minute forces, a cantilever 6 having a probing tip 12 and a length ranging from 100 .mu.m to 200 .mu.m is preferably employed in the AFM. When a sample 21 placed on a sample platform 4 is brought near the probing tip 12, the cantilever 6 deflects in the presence of interatomic forces existing between the probing tip 12 and the sample 21. The AFM scans the surface of the sample 21 using first and second piezoelectric members 1 and 2 and a piezoelectric member drive unit 11 while a third piezoelectric member 3 is being feed-back controlled by the piezoelectric member drive unit 11 via a control signal generator 14 so that the amount of deflection of the cantilever 6 may be maintained constant. The first, second, and third piezoelectric members 1, 2, and 3 are secured to and extend from the sample platform 4 in directions shown by arrows X, Y, and Z, respectively. Because the control quantity in such a feed-back control is indicative of height variations of the sample surface, an AFM image is obtained by converting the control quantity into image information using a controller or computer 10. Alternatively, the AFM image can be obtained by converting the amount of deflection of the cantilever 6 into image information without performing the feed-back control. The amount of deflection of the cantilever 6 is detected by a deflection detector 22 wherein the principle of optical beam deflection, laser interference, tunnel current, or the like is utilized. The resolution of the AFM depends upon the radius of curvature of the probing tip 12. The less the radius of curvature, the higher the resolution At present, an atomic image of, for example, mica is observed by a probing tip having a radius of curvature of several hundred angstroms. The AFM is used to observe not only a sample surface on an atomic scale but also another sample surface having relatively large height variations in unit of nanometers (nm) or micrometers (.mu.m). The observation of the sample surface of the latter, for example, a grating having relatively deep grooves, requires a probing tip having a small radius of curvature and a high aspect ratio sufficient to reach the bottoms of the grooves. In this respect, a whisker crystal is preferably used as the probing tip.
As a matter of course, in applications where the probing tip 12 is scanned on the surface of the sample 21, the cantilever 6 deflects in the presence of height variations of the sample surface. In addition to such deflection, variations in the friction coefficient of the sample surface or a distortion of the cantilever 6 causes a deflection of the cantilever 6. This kind of deflection brings about noise, and hence, accurate measurement of the surface configuration cannot be expected. By way of example, when mica is used as a sample and a repulsive force of 1.times.10.sup.-8 N is chosen to act on the cantilever 6, an AFM image obtained from the amount of deflection of the cantilever 6 indicates a generally symmetric atomic image. On the other hand, when a repulsive force of 1.times.10.sup.-7 N is chosen to act on the cantilever 6, the AFM image indicates a non-symmetric atomic image.
Furthermore, in applications where measurements are carried out using a probing tip having a high aspect ratio, and if the sample 21 contains very steep height variations or has grooves with generally vertically extending side walls, a side surface of the probing tip 12 occasionally collides against the side walls of the grooves during scanning. Under such conditions, little deflection of the cantilever 6 would occur, and hence, the scanning is continued with the distance between the sample and the probing tip 12 maintained substantially constant. As a result, not only can no accurate AFM image be obtained, but also the probing tip 12 or the cantilever 6 is occasionally damaged.
In addition, if the sample 21 is a living body which cannot be easily anchored on a substrate, the scanning of the probing tip 12 drags the sample 21 on the substrate, thus resulting in an inaccurate measurement.